Method and apparatus for making lightweight molds



Aug. 27, 1957 R. J. COWLES ETAL 2,803,859

METHOD AND APPARATUS FOR MAKING LIGHTWEIGHT MOLDS Filed April 12, 1955 IN VEN TORS PHIL/P 5E55/N67'0A/ PODEEMK Con/45.5 M/C/MEL flue/rzm l United States atent -O METHOD AND APPARATUS FOR MAKING LIGHTWEIGHT MGLD?) Roderick J. Cowles, Needham, and Michael Dubitzky, Stoneham, Mass, assignors, by mesne assignments, to Lebanon Steel Foundry, Lebanon, Pan, a corporation of Pennsylvania Application April 12, 1955, Serial No. 500,928

11 Claims. (Cl. 22-9) This invention relates to a method for making lightweight molds over objects of a predetermined configuration, and more particularly to molds for casting metals.

In the metallurgical industries, metal castings can be made by a number of methods including casting in sand molds, in permanent molds and in resin bonded sand molds generally called shell molds. The method chosen is determined by such factors as shape and size of the castings to be made, metals to be cast, temperatures to be used, etc. Where large-scale production of relatively small castings is required the use of shell molds has been found advantageous particularly where bronze, brass, carbon steels, and alloy steels are cast. Briefly, the shell molding process is based on forming a thin-walled, resinbonded, sand mold or shell mold on a hot metal pattern. The metal pattern at aboutASO-f P. is suspended in a dump box and the box is rotated through one hundred eighty degrees to dump the resin-bonded sand on the hot pattern. The sand-resin mix is left in contact with the pattern until sufficient heat has been transferred to the mix to build up a fused resin bonded sand shell of desired thickness. The pattern with the built-up shell is removed from the dump box and transferred to an oven for final curing. It is usually necessary to support the mold with metal shot while casting. Since the shell molds made by this process are usually of the order of one-fourth to three-eighths inch thick, the weight of the final casting is somewhat limited, usually to about fifty pounds. More important, however, is an inherent limitation on the bulkiness of the final casting.

The shell molding process hasa number of important advantages among which are ability to precisely re-produce the pattern surface, lightnessin weight of the shell molds, ease and rapidity of making the molds, and attainment of excellent surface characteristics of the final casting.

While these so-called shell molds made in the manner described above have distinct advantages and are finding widespread application, they also have certain limitations. Among the more serious limitations and disadvantages of shell molds made in the dump box are the necessity of using a metal pattern which makes the method too costly for small runs and particularly for larger molds, the necessity for machining metal patterns regardless of size, and the limitation on shell thickness and hence on the bulkiness of the final casting. In addition, the dump-box method is inherently time-consuming.

A process has been described in which resin and sand are sprayed onto a pattern by means of an air gun or the like, with the mixture being heated to form a mold on a pattern (German Patent No. 832,935 issued to Croning). close means for making usable shell molds of dimensionable stability and apparently does not take into account the stringent requirements of metal casting required in up-to-date foundry practice.

By the method of this invention it is possible to make shell molds or even solid configurations, such as cores,

The method as outlined, however, fails to dis- 2,803,859 Patented Aug. 27, 1957 ice possessing all of the advantages of the previously made shell molds without their disadvantages.

An object of the present invention is to make lightweight molds by projecting heated resin-refractory grain streams on patterns and bonding the resin-refractory mixture as it impinges on the pattern, and at the same time etfecting the partial curing of the resin on thepattern. it is a further object of this invention to make such molds by projecting a mixture of a thermosetting resin and refractory grains from a gun arrangement with- .out clogging the exit of the gun while at the same time raising the temperature of the thermosetting resin to a plastic stage after the mixture passes through a predeterfi1lI1d1thTOW distance. It is another object of this invention-to make molds suitable for metal castings on wooden or other inexpensive patterns. It is still a further object of this invention to make such molds with great rapidity. It is still another object of this invention to form such resin-bonded refractory grain molds on a pattern with substantially complete dimensional stability; It is still a further object of this invention to make a mold which is readily removed from the pattern without reference-to the size of the mold. It is an additional object of this invention to make shell molds capable of being used for making metal castings of much greater bulk than are possible with shell molds madeby other processes. It is still another object of this invention to provide an apparatus suitable for making such molds. These and other objects will be shown to have been .accomplished in the following description of this invention.

The method according to the present invention is based on the finding that strong, coherent, lightweight molds or objects over or in predetermined configurations can be formed on inexpensive patterns made of wood orthe like by projecting a stream of refractory grains in close relationship with a thermosetting bonding agent into a substantially uniformly heated high velocity gas stream, transporting and partially heating, in a predetermined manner, the bonding agent and refractory grains :by means of the gas stream, projecting said refractory grains and bonding. agent across an unconfined, predetermined throw distance, through which throw distance additional heat is transferred to the bonding agent and refractory grains by the gas stream or by auxiliary heat sources, onto a pattern where the refractory grains are bonded 'by the bonding agent to form a mold. This mold is cured at least in part while on the pattern. The stream of refractory grains and bonding agent carried in the hot gases may be so directed as to build successive (usually adjoining) sections of the shell to the desired thickness and/or to build up the shell in successive layers. The first method is preferred for operational reasons.= By forming successive sections of the mold each portion thereof will cool somewhat while adjacent sections :are being sprayed and the formation of any substantial cooling strains will be minimized or completely eliminated.

The use of inexpensive patterns made from wood, treated wood, filled phenol and filled epoxy resins, or other suitable materials, makes the shell molds made by this invention applicable to short run castings, even for large molds. In addition, the use of wood or other material for patterns eliminates the expensive machining .of a metal pattern. Since the process of this invention is independent of the heat transfer between the mold and pattern, the shell mold thicknesses may be readily controlled and built up to any desired thickness. This also means that shell molds may be made with great rapidity by the process of this invention as compared with the dump-box method where the hot pattern must remain in contact with the resin and sand for a fixed period of time. The ability to make thicker shell molds which are cured or partially cured in formation makes it possible to use shell molds for operation.

much bulkier castings than previously possible. The fact that the refractory grains and bonding agent are projected against the pattern with some force means that the grains will enter into narrow crevices and corners and conform to any shaped contour of the pattern making it possible to make shell molds for intricate castings. The manner in which the shell is built up, i. e., by building up portions of the mold, minimizes cooling strains and hence means the attainment of good dimensional stability.

In making the improved shell molds of this invention it has been found essential to apply suflicient heat in a controlled, predetermined manner to the refractory grain- .resin mixture as it is being built up on the pattern to bring the resin to its fused stage. This is necessary to obtain bonding, to prevent rebounding on arrival at the pattern and to convert at least a part of the resin into an infusible, irreversible solid state while at the pattern to assure dimensional stability and rigidity on subsequent application of heat, such as in an after-curing stage or in the casting This is done preferably by having the hot gas stream carry the refractory grains and bonding agent at a temperature and in a sufficient amount to condense the bonding agent on the pattern at a rate which will convert at least a part of the bonding agent from the fusible state to an irreversible state. This means that advantage is taken of the property of such resins that the rate at which the bonding agent is advanced to the irreversible, infusible stage will lag slightly behind the rate at which the bonding agent is fused or becomes tacky.

Generally speaking, as in any chemical reaction, the rate of curing or of condensing the bonding agent to its final irreversible stage will increase with increasing temperature. Therefore, if the resin reaches the pattern at a temperature at or slightly above its softening temperature it will remain in its fusible state for a longer time than if it had reached the pattern at higher temperatures. For various configurations of equipment for applying the refractory grain bonding agent mixture the temperature conditions, as well as the ratio of hot gas to the said mixture, can be readily determined experimentally by those skilled in the art.

Some resins which are not so readily advanced to the irreversible stage (either because of their nature and/or because of the fact that they have been advanced less before use in the process) may need a short pass of the hot gas stream on the back side of the mold after the entire thickness has been built up. It is feasible, but less desirable, to break up this process into steps by applying the shell in several layers whereby any or each of these layers may be advanced to the desired rigid conditions by hot These critical operating parameters may best be presented with reference to Fig. 1 which represents a diagrammaticsketch of a gun arrangement suitable for carrying out this invention. In Fig. 1, liquid or gaseous fuel enters chamber 1 through line 11 and is burned in a primary air stream entering through line 12. Secondary air, admitted through line 13, is used to reduce the temperature of the gases to that required for the hot gas stream. The resulting hot gas stream then leaves chamber 1 and passes through duct 2 into a channel 3, and through an opening 6 into a cylindrical or tubular passage 8. The refractory material mixed with or coated by the resin or bonding agent is carried under pressure in a conduit such as line 4 through exit 7 into the cylindrical passage 8 which serves as a zone for dispersing the refractory grains and bonding agent in the hot gas stream. Line 4 contains a check valve 14 and is encased by a thermal shield or insulation 5.

The mixture of hot air with the refractory grain-resin mixture leaves the gun exit 9, passes through the unconfined throw distance designated A and impinges on the pattern 10. The cylindrical passage 8 has a length designated 0. Although the particular embodiment of this invention illustrates the passage 8 to be cylindrical, it may be designed to have other suitable cross sections. Cylindrical passage 8 has an exit inside diameter designated D.

In order to provide flexibility of arrangement with respect to the relationship of the pattern 1! to the exit 9, the gun may be suspended at a suitable point by suspending means 19. Alternatively, it may be mounted on a universal joint arrangement, supported for example on the floor, which would permit aiming the gun through a vertical and horizontal plane. Additional flexibility of movement in the suspended arrangement shown in Fig. 1 may be achieved by having the gun move in relation to duct 2. The pattern It is mounted in a suitable supporting means 15. These supporting means, in turn, are held in a rocker arm 17 which is mounted on base 18. The gimbal mounting 16 permits the pattern 10 to be moved up and down and rotated around its horizontal and vertical axes, while the rocker arm permits the pattern to be moved towards and away from the gun.

The design shown in the figure illustrates one possible embodiment of the process according to this invention. This invention is, however, not limited to such a scheme. Design variations apparent to those skilled in the art and within the scope of the claims herein may be readily devised. Thus, for example, the hot gas stream may be provided by heat exchange of air with electric heaters, or by heat exchangers without direct utilization of combustion gases. The passageway may be slightly conical in section With the Wider end being at the exit. Auxiliary heating of the gases, or auxiliary heat to the gases may be furnished near or at the exit of the passageway. Many other variations will be apparent to those skilled in the art once the general principles of this invention are fully understood.

The critical operating parameters in this system are the diameter D of the gun exit, the length 0 of the cylindrical passage 3, the distance A the mixture must travel from the exit 9 to the pattern 1t), the temperature to, and velocity 1 0 at which the gas stream leaves the gun exit 9 and the temperature t and velocity v at which the hot gases, containing the refractory material and resin, strike the pattern it).

Since the above parameters are interrelated, as will be shown hereinafter, it Will be seen that the gun design and dimensions must be integrated with the operating conditions, i. e., temperature and velocity, and with the location of the pattern. Thus, to obtain consistently superior shell molds made by the process of this invention, it is necessary to provide controlled operating conditions within limits which would not be ascertainable to one skilled in the art of shell molds as previously known without the teachings and equipment described herein.

Although gun arrangements have been used for ejecting a stream of solid matter carried in a gas, and although some of these arrangements have used heated mixtures which cool and adhere to an object after impinging on the object, the process of this invention presents a problem which appears to be new, in that dealing with thermosetting materials it is required that the materials deposited on the target or pattern are hotter than when they leave the gun arrangement which forces them against the target. Such operating conditions not only eliminate clogging in the gun apparatus but also permit the establish-- ment of a heating schedule for the bonding agent across the throw distance and on arrival at the pattern. It is,

therefore, an essential feature and unexpected result of the procedure of the present invention that material such as the resin-coated refractory grains or resin-refractory grain mixtures leave the gun exit at a temperature considerably lower than that finally required and are heated sufficiently in the throw distance A to attain a desirable tackiness or flowability andthen, at least in part, to set permanently ,after impingement.

Before presenting a quantitative analysis Oflthfi factors involved a qualitative discussion of the interrelationships noted above will be given. In the following presentation the mixture of refractory grains and bonding agent will be referred to as the resin-coated refractory grains. However, :it is to be understood that the bondingagent may be physically mixed with the refractory grains, although the use of such resin-coated refractory materials has been found generally more preferable.

With reference again to Fig. 1 heat is furnished to the system by hot gases which pass from the chamber 1 through duct 2 and channel 3, into the cylindrical passage 8 where dispersion and a preliminary transfer of heat from the hot gases to the resin-coated refractory material take place. The resin-coatedrefractory material as it passes through line 4 must be kept below the temperature at which theresin willsoften or become sufliciently tacky to formgagglomerates which would clog line the softening point of the resin or to a temperature highenough to cause the material toset on thegun walls or at the exit 9. This temperature limitation in turn is interrelated with the type and quantity of resin-coated refractory material used, the velocity of the mixture and also the length c of the cylindrical passage 8 wherein mixing takes place. The temperature of the resin-coated refractory material at the gun exit must notbe so great as to convert the resin to its plastic or tacky state or to its setting temperature (so-called C-stage for thermosetting resins) and hence to build up a coating around the exit 9, or to advance the resin to a degree that it will be incapable of bonding the refractory material once it strikes the pattern surface. The temperature of the resin-coated refractory material at the pattern surface must, however, be raised to the point where the resin is plastic and flowable and sufiiciently tacky to bond the refractory grains as they strike the pattern. This temperature may be such as to effect the curing of the resin as the mixture of resin and refractory grains strike as long as the resin has not been advanced beyond its tacky stage. This means then that the resin-coated refractory material must be heated while being carried through the throw distance A.

Such relatively stringent temperature requirements place equally stringent requirements on the velocity of themixture. In the cylindrical passage 8 of the gun muzzle the velocity must be great enough to prevent the resin-coated refractory material from adhering to the sides and also high enough to carry the resin-coated refractory material in suspension. The mixture velocity mustbe high enough also to prevent clogging at the exit 9. During the throw distance A the velocity must not be so low as to permit some of the resin-coated sand to fall out or to permit any substantial cooling by the outside aemosphere. On the other hand, the velocity must not be so high asto prevent the accomplishment of the necessary heat transfer to raise the resin-coated refractory material from the temperature at which it is ejected from exit 9 as it passes through the throw distance A. In addition, the mass-velocity of the mixture as it strikes the pattern 10 shouldbe such that a shell of the required density may be. built up from the resin-coated refractory material. This means that the flow rate of the resin-coatcdrefractory grains must be integrated with the velocity of the mixture. A final restriction on the mixture velocity is that it must not be so great as to result in the resin-coated refractory grains bouncing off the pattern as they strike the surface.

The requirements that a certain amount of heat transfer must be accomplished .over the distance A means that .A is determined by the gas temperature to (which in turn determines the actual temperature of the resin coated refractory materialcarriedin the gas as it leaves exit 9) and by the temperature: which represents the temperature at which the hotgasstream must-strike the pattern to :raise the resin to the required temperature. The velocities v0 and v also enter directly into-.the-determination of A. Since the final velocity v at the pattern is directly related to the diameter of the exit D, this parameter, i. e., the exit inside diameter D, must be determined with regard to the velocities employed. This exit diameter must be great enough to prevent clogging of the gun exit 9 by the mixture, but small enough. to

.keep the final velocity up to the desired level.

Thus, from the above qualitative discussion it will be seen that criticalinterrelationships exist among c, D, A, to, I, vs, v, and the resin-coated refractory grain flow rate. These relationships may be evaluated and stated quantitatively ffor any-type of resin used. Thus, the various design parameters may be determined for a given resinrefractory material ratio. This may best be explained first by reference to .a specific example, namely, that in which sand, coated with about 4.5% phenol formaldehyde resin, is used. Then more general definitions may be given in terms of resin .performance.

The phenol formaldehyde resin of this example has a softening temperature range of about to 220 F. and a setting temperature range of about to 250 F. in which range theresin is also cured. (Curing is actually a function of time and temperature, while the so-called setting temperature range is that temperature range within whichthe resin will advance to its irreversible stage.) Temperatures above 5 00 F. for extended periods shouldbe avoided to prevent charring of the resin. This means that t, which represents the temperature of the hot gas stream necessary to raise the resin to its desired temperature should probably not exceed 500 F. while the actual temperature of the resin should preferably not be above about 350 F. Since, as pointed out above, the resin-coated refractory material at the exit 9 of the mixing zone should not be substantially above the softening temperature of the resin, and the final. heat transfer must take place over the throw distance A, the temperature of the resin-coated refractory material at exit 9 should not be above about 150 to 220 F. while the gas stream from which the heat is transferred, should preferably be in the range of 500 to 900 F.

Under these conditions it is possible and desirable to maintain high velocities in the gun, namely between about 2000 and 6000 feet per minute. By determining the relationship between impact velocity and temperature, v and t, and gun discharge velocity and temperature v and t a limiting design range is established which will consistently produce the best overall shell molds.

Under operating conditions the relationships of. v/v and t/t to A/D have been experimentally determined. These relationships may be generally expressed as m m-0.06 A/D and At/At -10.075 A/D Where v and v represent the velocity of thehot gas stream at the pattern and at the gun exit, At and Ar represent the difference between the ambient temperature and the gas temperature at the pattern and at the gun exit, and A and D are the throw distance and the exit inside diameter, respectively. These equations represent general rather than exact relationships, the latter not being necessary for an apparatus set-up such as: this.

Since the temperature limits of the hot gas stream are determined by the characteristics of the resin used as outlined in the above example, the ratio At/At (and hence A/D is necessarily limited. Practical considerations, such as allowances for variations in the depth of the pattern contours and effect of outside cooling of the hot gas stream, limit the throw distance A which in turn limits the exit diameter D. These limits on A and ,D,

over the throw distance.

- along with the heat transfer properties involved, and the required minimum impact velocity, all narrow the design to a specific operable range. a

The following conditions were found to be the practical limits constituting this invention and are basic to any device which is designed to use the method of this invention.

For a reasonable rate of shell production the temperature at which the resin-coated refractory grains impinge on the pattern should be near the upper limit at which the resin chosen can be applied and cured. That is, the resin should be in a plastic and flowable condition capable of bonding and curing. Thus, in the case of a phenol formaldehyde resin as used in the example cited, the minimum temperature of the hot gas stream at the pattern, 2, should not in practice be below 250 F., and should not be above about 500 F. while the maximum temperature of the hot gas stream at the gun exit should be about 900 F. Assuming an ambient temperature of 80 F., a maximum A/D ratio can be calculated to be Setting a minimum average throw distance A of 12 inches and a minimum of 10 inches for any one point on the pattern (based upon the practical considerations stated above) the minimum gun exit diameter at the nozzle is about 1.5 inches, based upon the minimum average throw distance. Experimental work has shown that the gun exit diameter D should not be more than to 6 inches if evenly formed shell molds and sharp definitions in the mold contours are to be maintained. A minimum value of about 2 for A/D can then also be established using the minimum average throw distance of 12 inches.

The velocity of the stream as it emerges from the gun nozzle must be kept at or above a minimum to protect the gun from coating with any cured resin-coated sand.

. A low velocity can be maintained only with low temperatures and low A D ratios.

Using the design limitations described above, the following ranges have been found operable for a phenol formaldehyde-coated foundry sand:

Gas temperature in gun at point of contact with resin-coated refractory grains F 700l400 Gas temperature at gun exit (t F 500 900 Gas temperature at pattern (t) F 250- 500 Gas velocity at gun exit (v F. P. M 2000-6000 Gas velocity at pattern (v) F. P. M 1000-5300 Length of cylindrical passage (0) feet 46 Gun exit inside diameter (D) inches 1.5-6 Throw distance (A) inches 1060 A/D 2-l0 While this does not form a primary part of this invention, best results will be obtained when the following conditions are also maintained:

Grade of sand 40l60 AFS 1 Amount of resin, by weight of sand-resin mixture M 1-10 percent. Shell mold thickness As-l inch.

'lhese figures are used to designate sand grain sizes and indicate the standard mesh size of the largest portion of sand in a particular lot. Thus 80 AFS sand will contain grains larger and smaller. than 80 mesh but have the greatest portion of grains close to 80 mesh,

From these ranges a number of different sets of optimum operation conditions can be chosen. However, it

'must be kept in mind that the relationships set forth above must be respected so that, for example, if high gas velocities are chosen, the higher gas temperatures must be used to achieve the required heat transfer to the resin The rates of fiow of the primary and secondary air, of the fuel and of the resincoated refractory material are determined experimentally to give the desired temperature and velocity conditions.

For a specific example embodying the process of this invention, a list of a typical set of optimum operating conditions using phenol formaldehyde resin-coated sand is given below:

Primary air flow rate 45 C. F. M. at

Secondary air flow rate C. F. M. at

Fuel gas flow rate (propane) 2.4 C. F. M.

Gas temperature in gun at point of contact with resin-coated refrac- Grade of sand 140 AFS. Amount of resin, by weight of sand-resin mixture 4.5%. Flow rate of resin-coated sand 46 lbs/min.

Shell mold thickness through thinnest section.

Final curing on pattern Shell back cured with hot air from gun.

After-cure Placed in oven at The operating conditions and design parameters suitable for making shell molds by the process of this inven tion may be stated in general terms using resin performance as a basis of reference. The final temperature t at which the hot gas stream carrying the resin-coated refractory material strikes the pattern should be such as to impart flowability and plasticity to the resin and also to heat the resin suficiently so that it will set at least in part, and bond the refractory grains as they strike the pattern. Since curing is a function of time as well as of temperature, there can be a range for the temperature of the gas stream at the pattern if sufiicient heat is kept in the shell for final curing or additional heat is added for a separate curing step. However, it is essential that the resin is advanced to the stage where immediate bonding of the refractory grains takes place and negligible bounceofi of the grains occurs. Thus the lower limit of t is that gas temperature which will impart suflicicnt heat to the resin to bond the refractory grains as they strike the pattern while the upper limit of t is that gas temperature which is just under that which would cause the resin to char. As a rule, the temperature to bring about fiowability and/or plasticity will, if maintained for a sufficiently long time, be sufficient to convert, at least in part, the resin into its irreversible, hard stage (C-stage for phenol formaldehyde resins).

By varying the hot gas temperature t within the range described above it is possible to control the amount of final curing achieved in the shell mold. Thus, if t is kept consistently at the higher levels, the shell mold will be more completely cured as it is built up. Conversely, by using the lower levels of t, the shell mold will be only partially cured, i. e., additional heating will be required completely to convert the thcrmosetting resin to its irreversible stage. An alternative is to vary the temperature of the hot. gas stream or the amount of resin-coated refractory grains carried by it so that sections or layers of the shell mold are completely cured. Thus, it might be desirable to cure the inside or outside layer (if the shell mold is built up in layers) to give the shell mold sufiicient structural strength to be placed into an oven or further heated to effect final curing. The attainment of such added strength prevents :sagging of the rshellmoldyif icured further. Although the shells may, as indicated, he built up in layers, it has been found preferable to build up successive seetionsto their complete thickness thus effecting at least parti al curing throughout the entire shell mold. when -this is done an additional pass of the hot stream, without the .resinecoated refractory grains, over the back side lOf the shell moldmay be desirable more completely to cure the outside; to give .the shell. mold dimensional stability.

'The temperatureof the hot gas stream to as it leaves the exit 9 of the cylindrical passage .8 should preferably be that which will have raised the resin-coated refractory grains. to a temperature which is equal to or not substantially in excess ,of the softening point of the resin. If to falls greatly below this temperature, the throw distance will have to be extended to give more time for the heat transfer process between the exit 9 and the pattern 10. If it is ,raisedabove this limit, the resin will have advanced too far so that it will permanently clog the exit 9 and probably will have passed the plastic or fiowable stage so that it can no longer effectively bond the refractory grains as they strike the pattern.

Practical operating considerations will limit v to between 2000, and 6000 feet per minute, and A/D to between about 2 and .10. Adjustments can be made in operating conditions and design parameters so that the processjof this invention can operate beyond these ranges but optimum results in the form of consistently superior shell molds will, be obtained by using these'practical ranges.

In a typical arrangement illustrated diagrammatically inFig. l, the relationship between gun nozzle and pattern maybe maintained by fixing the gun and moving the pattern, or fixing the pattern and moving the gun, or by a combination of these two. The pattern is mounted in a'supporting means which in turn is fixed to a gimbal mounting 16 which permits the pattern to be rotated around its horizontal and vertical axes. While the arrangement in Fig. 1 shows one possible arrangement for moving the pattern in relation to the gun exit, a number of other equally feasible arrangements are possible and Will be evident to one skilled in the art.

As pointed out, it may be desirable to move the gun in, relation to the pattern. This may be accomplished by any one of a number of means such as the suspension means 19: or by mounting it on a universal joint arrangement supported, for example, on the floor. Of course, if the apparatus comprises both a movable gun and a movable pattern, it may be desirable to move both.

Finally, the length 0 of the cylindrical passage 8 can be shortened or lengthened by sliding the walls of 8 in and out over the walls of channel 3 by means of a telescopic arrangement, for example, or by having interchangeable passages of different lengths. Changing the length 0 of the cylindrical passage permits adjusting the design and operating parameters for resins with varying character istics. Thus, for example, if it were desirable to use a resin or bonding agent with a low softening point, it would be necessary to shorten 0 so that less heat would be transferred in the cylindrical passage 8 and to kept to the softening point or below to prevent clogging exit 9. If c were shortened A would automatically be increased, a necessity in this case since a greater part of heat transfer might have to be accomplished during the throw distance A to raise the resin to its pastic stage or curing temperature by the time it reaches the pattern 10.

In addition to making the operating conditions easily adjustable, it is desirable to be able to move the gun or pattern or both in order to avoid entrapment of accientally unfused resin-coated refractory grains which may be caused if the pattern is so placed that such refractory grains cannot fall off. Thus, it may be preferable to have the patternin an inverted position, that is, with its surface pointing more or less downward, its base being at an angle preferably between 45 and 180 to the horizontal gether, if desired after insertion of a core.

plane. In this manner any resin-coated grains haying insufl'leient bonding tackiness will be permitted to bounce or fall off without impairing thestrength of the resulting mold. In the layer build-up process, or where the. resin is such as to make mass application ,of the resin-coated refractorygrains undesirable, .then it will be preferable to have the pattern facing downward. However, where the flowability and tackiness of the resin permit application of the resin-coated refractory grains as a mass, no problem of bounce-off or fear of crevices with unfused refractory grains is present and the pattern may be used facing upward.

If the shell mold is built up in layers, then the pattern can be returned to horizontal position after the first layer has been built up to facilitate and accelerate building up of the back-up layers. After each successive pass over the pattern, each layer thus formed may be advanced to an irreversible stage by further application of heat, or selected layers may be so advanced to become substantially infusible, thus reinforcing the whole shell. Alternatively, the hot gaseous stream can :be made to form a hot envelop around the refractory grain-resin stream and advantage can also be taken of the fact that curing of the resin takes a finite time depending on temperature, so thateven with the mixed combustion gas-air stream being substantially above fusing temperature, the resin still will be in its fusible stage on reaching the pattern while advancing further afterarrival at the pattern.

In anycase, it forms an important part of our invention that the molds are at least partially cured before being removed from the pattern in order that they are self-supporting and dimensionally stable throughout the subsequent curing process and final use in metal casting. Of course, complete curing on the pattern. is possible if the resin is sufficiently advanced when it strikes the pattern. The heat required for curing may be conveyed by separate equipment or by the same equipment used for depositing the resin-coated sand on the: pattern, i. e., the resin-coated sand supply can be cut oif or reduced in flow, by manipulating valve 14, thus allowing the hot gases to impinge on the shell layer already formed.

The pattern is preferably coated with a suitable parting agent such as spraying it with a silicone emulsion and perhaps subsequently dusting it with a material such as lycopodium.

The mold formed in this manner is stripped off the pattern and may be transferred into a curing oven, where it is heated until the resin is sufficiently cured to the C-stage for giving the entire mold the desired rigidity. The mold thus formed may be handled as in the shell molding process orany other convenient process. Thus, the mold surfaces may be coated with customary mold washes. The shell halves may be glued or clampedto- When desired, gates and risers may be made in a "separate step and attached by suitable means. The mold assembly then may besupportedby metal :shot, or sand, such as waste foundry sand, and themetal cast into it in the usual manner.

Although, sand is generally preferred as the refractory material in foundrymolds because of its refractory properties, availability and economy, other refractory grains such aszircon, grog, etc, may be used. Choice of the material will dependrnainly on local conditions and met=allurgical requirements. Consistent with general foundry practice, such refractory materials will be used in relatively .fine grain size. While the selection of grain size and grain size distribution does not form part of our invention, proper selection of grading will be important with respecttoboth the density and the strength desired. Generally speaking well graded materials will give dense molds. However, where strength with relatively high porosity is desired, relatively coarse particles of uniform particle size may be used,,since such coarse particles will require less resin binder for a given weight than finer ones. Of course, it maybe desirable to vary the particle ment.

size 'and/ or distribution when building up the molds according to this invention, such as by using a thin fine grained layer at the inside, and coarse material for good venting as a back-up. Alternatively, the inside may be of coarse material. The structure, that is density and porosity of the mold, can of course also be controlled or varied by adjusting the velocity of the resin-sand streams and by control of the apparent density of sand streams.

As previously mentioned, the resins used should be so-called thermosetting resins, that is resins which after final cure are irreversibly hardened. This is of importance since foundry molds are subjected to high temperatures during casting and must retain rigidity until at least the metal adjacent to the mold surface has solidified. After such solidification it is, however, desirable that the molds do not lose their strength due to the further temperature rise during casting and that they offer strong resistance to the shrinkage of the metal on cooling so they can be shaken off readily. Obviously, therefore, thermosetting resins now used in conventional and shell molding foundry practice will be particularly suitable for the purposes of the present invention. In any case the suitability of any particular resin can be readily determined by experi- Selection of any particular resin will also depend on the specific equipment used and the compatability with the metals or alloys to be used in the casting process. Generally speaking, however, we prefer phenol formaldehyde or the related resorcinol or cresol formaldehyde resins in the so-called B-stage, which upon heating are 'capable of going through a thermoplastic (fusible) stage before finally going to the C or irreversible, finally cured stage. Usually they will contain or be blended with additional catalyst such as hexamethylene tetramine to accelerate curing during the application process previously described. However, other thermosetting resins which go through a tacky stage by means of the addition of catalysts or accelerators, such as urea formaldehydes, or by loss of solvent (water for example), such as melamines, may be used if desired. Furan derivative resins, core oils and many others will be representative of the thermosetting, resinous substances suitable for the present invention. In other words, thermosetting resins suitable for the present invention are those which upon the application of heat are capable of going through a tacky stage before finally going into irreversible soild resins. stage the resins preferably will be free from solvents, or as in the case of melamines they should contain only small residual amounts sutficient to give them a gel structure.

The practical and economical range of resin content is i from about 1 to 10 percent by weight of the refractory grains. However, it is preferable to keep the resin content below 6% since the lower percentages of resin will reduce gasification of the resin to a minimum during the early casting stages and will also reduce the cost of the r mold. Among the controlling factors, besides gas formation and cost, are the sizes of grains and the degree of bonding desired. Actually, there are no real upper limits on the amount of resin used from an operating point of view. However, the practical limits stated above should be considered. It is, in fact, a particular advantage of the process according to this invention that strong molds can be made with substantially smaller amounts of resin than in conventional processes. As mentioned previously, the use of these resins as a coating on the refractory grains is particularly advantageous for the present invention, since the resin-coated refractory grains permit excellent heat economy and help prevent overheating of the patterns.

It may be desirable to include one or more additives in the mold-forming mixture to modify the properties of either the resin or the refractory grains. For example, the addition of a thermoplastic resin such as polyesters, acrylics, vinyls, etc., or of a thermosetting resin having a thermoplastic stage at a temperature different from the in the tacky primary thermosetting resin could be used to modify the tackiness, and hence the bonding properties, of the resin.

In general, the pattern material is preferably one which has a low heat conductivity and exhibits good resistance to erosion and to temperatures up to about 500 F. for the short periods of time involved. While standard wood patterns are perfectly suitable, We have found that such materials as wood coated with a heat-resistant silicone phenolic varnish, wood impregnated with a resin such as a phenol resin and then, if desired, further coated with a silicone-phenolic varnish, cast phenolic resin, cast epoxy resin, or filled phenolic or epoxy resins are particularly suitable for patterns. Of these, patterns made of filled epoxy resins appear preferable since they exhibit good resistance over long periods of use and are also easily duplicated by standard molding techniques. Metals possessing relatively low heat conductivity or suitably insulated by resinous or other coatings would also make satisfactory patterns. High heat conductivity of the pattern material would not exclude such material for patterns, but this property would tend to deprive the shell mold, while being built up, of the heat which should be retained in it for finally advancing the resin to its irreversible stage and perhaps partial curing.

Our invention as herein described is, of course, not limited to the examples given or to the materials enumerated. Rather, our invention relates to an improved process which permits the use of thermosetting materials to bond refractory type grains (or similar material) to form a predetermined configuration such as a shell mold or a solid object. By the present invention, which provides for part of the heating of the thermosetting material to take place after it leaves the gun exit, thermosetting materials may be used without clogging the apparatus and at the same time are sufiiciently heated to bond refractory particles at a point beyond the gun exit.

The process of this invention is particularly suitable for making shell molds for metal castings. Shell molds made by this process possess all of the advantages of shell molds made by previously known methods. In addition the process of this invention has added advantages, among which are the use of pattern materials such as wood and filled resins, the ability to make such shell molds with great rapidity, the ability to better control shell thickness of the mold, and the possibility of making final castings of greater bulk than previously possible.

We claim:

1. The method of making resin-bonded refractory molds for the casting of metals comprising the steps of feeding refractory grains in close relationship with a thermosetting bonding agent into a substantially uniformly heated high velocity gas stream, transporting and partially heating said bonding agent and refractory grains by means of said gas stream along a passageway and projecting said refractory grains and bonding agent from the exit of the said passageway across a throw distance, through which throw distance additional heat is transferred to said bonding agent and said refractory grains, onto a pattern where said refractory grains are bonded by said bonding agent to form a mold, said bonding agent at said exit being at a temperature not greater than the softening temperature of said bonding agent and at the pattern at a temperature sufiicient to advance said bonding agent to a tacky stage as it strikes said pattern and to convert a sufiicient part to its irreversible stage to provide a mold which is self supporting and dimensionally stable on being removed from said pattern, said gas stream having a velocity at said exit ranging from about 2060 to 6000 feet per minute, the length of said throw distance being not more than about 10 times the inside diameter of said exit of said passageway, said gas stream being at a temperature between about 500 and 900 F. at the exit of said passageway and at a temperature between about 250 and 500 F. on the pattern surface.

2. The method of making resin-bonded refractory molds for the casting of metals comprising the steps of feeding phenol formaldehyde resin-coated sand into a substantially uniformly heated high velocity gas stream, transporting said resin-coated sand by means of said gas stream along a passageway to the exit of said passageway while heating said resin-coated sand by said gas stream, during said transporting, to a temperature approximately equal to that of the softening point of said resin, and projecting said resin-coated sand from the exit of said passageway across a throw distance onto a pattern, said phenol formaldehyde resin being present in a concentration ranging from about 1 to 10 percent by weight of said sand, said high velocity gas stream having a temperature between 500 and 900 F. and a velocity between about 2000 and about 6000 feet per minute at the exit of said passageway, said gas stream continuing to heat said resin-coated sand during passage across said throw distance, and said continued heating and the time of traversing said throw distance being such as to convert said phenol formaldehyde resin to its plastic and tacky state and advance a sufficient part thereof to its setting temperature so that when said resin-coated sand strikes said pattern it provides a configuration which is self-supporting and dimensionally stable on being removed from said pattern.

3. The method of claim 2 in which said gas stream is at a temperature between about250 and 500 F. on the pattern surface.

4. The method of claim 2 in which the length of travel of said resin-coated sand along said passageway is between 4 and 6 feet, and said throw distance is between 10 and 60 inches.

5. In an apparatus for making resin-bonded refractory molds on a pattern for the casting of metals, means for heating a gas stream to a predetermined temperature, means defining a first passageway, closure means at one end of said first passageway, means defining a second passageway extending through said closure means and into and longitudinally of said first passageway and in spaced relation to said first passageway for introducing refractory grains and a thermosetting bonding agent in close relationship axially into said first passageway, heat insulating means surrounding said means defining said second passageway, and means for introducing said heated gas stream to said first passageway at a point between said closure means and the point of termination of said means defining said second passageway.

6. The apparatus of claim including means for 14 supporting a pattern at a predetermined throw distance from the open end of said first passageway.

7. The apparatus of claim 5 in which said first passageway slidingly engages means defining a third passageway extending about said first passageway.

8. An apparatus for making resin-bonded refractory molds on a pattern for the casting of metals, which comprises means for heating a gas stream to a predetermined temperature and for imparting an initial velocity thereto, means defining an elongated passageway closed at one end and open at the other end, means for introducing said gas stream into said passageway, means for introducing refractory grains and a thermosetting bonding agent in close relationship into said gas stream in said passageway for ejection through said open end thereof, means for controlling the length of said passageway between the point of introduction of said grains and said bonding agent thereinto and said open end, and means for supporting a pattern at a predetermined throw distance from said open end of said passageway.

9. An apparatus in accordance with claim 8, wherein the means for introducing the refractory grains and bonding agent consists of a thermally insulated tubular passage extending into said passageway, through said closed end thereof.

10. An apparatus in accordance with claim 8, wherein means are provided for effecting relative movement between said elongated passageway and said means for supporting a pattern.

11. An apparatus in accordance with claim 8, wherein said throw distance is between 2 and 10 times the diameter of the open end of said passageway.

References Cited in the file of this patent UNITED STATES PATENTS 2,059,983 Dent et al. Nov. 3, 1936 2,427,448 Duccini "Sept. 16, 1947 2,521,179 Mitchell Sept. 5, 1950 2,643,955 Powers June 30, 1953 2,683,296 Drumm et al. July 13, 1954 FOREIGN PATENTS 492,477 Canada Apr. 28, 1953 832,935 Germany Mar. 3, 1952 1,072,908 France Mar. 17, 1954 OTHER REFERENCES Phenolic Resins for Shell Molding Process, General Electric, 6 pages received in Patent Ofice June 3, 1952. 

